Conventionally, to perform communication only among specific persons, a method is adopted in which key information for encoding and decoding is shared between the transmitter and the receiver and an operation or an inverse operation of the information data (plain text) to be transmitted is mathematically performed based on the key information to thereby realize secret communication. FIG. 53 is a block diagram showing the structure of a conventional data transmitting apparatus based on the method. In FIG. 53, the conventional data communication apparatus is constituted by connecting a data transmission apparatus 90001 and a data receiving apparatus 90002 via a transmission path 913. The data transmission apparatus 90001 has an encoding part 911 and a modulating part 912. The data receiving apparatus 90002 has a demodulating part 914 and a decoding part 915. In the conventional data communication apparatus, when the encoding part 911 receives information data 90 and first key information 91 and the decoding part 915 receives second key information 96, the decoding part 915 outputs information data 98. Hereinafter, the operation of the conventional data communication apparatus will be explained with reference to FIG. 53.
In the data transmission apparatus 90001, the encoding part 911 encodes (encrypts) the information data 90 based on the first key information 91. The modulating part 912 modulates the information data encoded by the encoding part 911 in a predetermined modulation format, and transmits it as a modulated signal 94 to the data receiving apparatus 90002 via the transmission path 913. In the data receiving apparatus 90002, the demodulating part 914 demodulates the modulated signal 94 transmitted via the transmission path 913 in a predetermined demodulation format, and outputs it as encoded information data. The decoding part 915 decodes (decrypts) the encoded information data based on the second key information 96 shared with the encoding part 911, and reproduces the original information data 98.
Now, wiretapping by a third party will be explained by using a wiretapper data receiving apparatus 90003. In FIG. 53, the wiretapper data receiving apparatus 90003 has a wiretapper demodulating part 916 and a wiretapper decoding part 917. The wiretapper demodulating part 916 wiretaps the modulated signal (information data) transmitted between the data transmitting apparatus 90001 and the data receiving apparatus 90002, and demodulates the wiretapped modulated signal by a predetermined demodulation method. The wiretapper decoding part 917 tries to decode the signal demodulated by the wiretapper demodulating part 916 based on third key information 99. Here, since the key information is not shared between the wiretapper decoding part 917 and the encoding part 911, the wiretapper decoding part 917 tries to decode the signal demodulated by the wiretapper demodulating part 916 based on the third key information 99 different from the first key information 91. For this reason, the wiretapper decoding part 917 cannot correctly decode the signal modulated by the wiretapper demodulating part 916, and cannot reproduce the original information data.
The mathematical encryption (also called computational encryption or software encryption) technology based on such a mathematical operation is adaptable, for example, to an access system as mentioned in a patent document 1. That is, in a PON (passive optical network) structure in which an optical signal sent out from one optical transmitter is branched by an optical coupler and distributed to each of the optical receivers at a plurality of optical subscriber homes, a signal for other subscribers other than the desired optical signal is inputted to each optical receiver. Therefore, by encrypting the information data for each subscriber by using different pieces of key information, information of each subscriber is prevented from being leaked and wiretapped, so that safe data communication can be realized.    The patent document 1: Japanese Laid-Open Patent Publication No. H09-205420